Reflective electronic displays are a new type of display that is gaining popularity. For instance, reflective displays have been used in electronic book readers. In contrast to conventional flat-panel displays that require their own light sources, reflective displays utilize ambient light for illumination to generate the displayed images. Thus, reflective displays can mimic the look of “ink-on-paper” printed materials such as book pages or newspaper prints, and are often referred to as “electronic paper” or “e-paper.” Due to the use of ambient light for illumination, reflective displays have the significant advantages of lower power consumption compared to conventional displays, and the ability to be viewed under bright ambient light.
A major challenge in developing reflective displays is to provide good color without making the device structure overly complicated and difficult to manufacture. A number of reflective paper-like display technologies attempt to achieve satisfactory color by stacking several active device layers. Those techniques rely on subtracting, by absorption, a portion of the visible spectrum in each active layer and reflecting the remainder to create the desired color. Providing full color requires the ability to independently subtract portions of the incident visible spectrum corresponding to at least three primary colors such as RGB (red green blue) or CYM (cyan yellow magenta). In addition, a fourth layer is often required to enable black and white and/or provide grayscale. In some cases it may be possible to utilize fewer than four active layers to obtain full color because two primary colors may be achieved in one active layer. However, stacking even two active layers can be problematic. Multiple layers require multiple electronic backplanes and/or complicated vias. This leads to more difficult manufacture, lower yields, and greater cost. The use of multiple layers also degrades device performance. Stray reflectance at interfaces and absorption in the stacked layers due to less than perfect transparency reduce contrast and brightness.
An alternative to stacking active layers is to try to achieve full color through the use of sub-pixels, each of which provides a primary color by reflecting only a narrow band of that color in the incident light. The problem with this approach is that only a small fraction of the pixel area is used for creating each primary color, and the incident light falling outside the sub-pixel is wasted. This leads to unacceptably low luminance levels. Also, some technologies, such as the front-back electrophoretic displays, have the problem that the “off” color is still partially visible when the “on” color particles are pulled to the front of the pixel. To date, no existing single-layer reflective display technology has demonstrated acceptable color.